doesn't really matter.. it all drills down to your personal preference
aesthetically, i prefer the 1/48 since they are more or less of the same "size" as the other mechas especially the recent releases. in a non-hobbyist sight (which generally doesn't have any idea at all as to what scale are the VF-1, 0, YF-19 etc. etc.) it looks more complimenting to see a display of different valkyries with the same sizes.